Porcine Reproductive and Respiratory Syndrome (PRRS) is a swine infectious disease caused by PRRSV (Porcine Reproductive and Respiratory Syndrome Virus). The clinical manifestations of the disease include reproductive disorders in sows and respiratory distress in piglets. Early stage symptoms include fever, lethargy, anorexia, listlessness, respiratory distress, cough and etc. The disease was first identified in North Carolina of the United States (in 1987), and it was subsequently found in Canada (in 1998), Germany (in 1990) and Britain (in 1991). The disease was named as Porcine Reproductive and Respiratory Syndrome (PRRS) by The World Organization for Animal Health (OIE) in 1992. The epidemic of PRRS was reported in China in 1996. PRRSV can be divided into two genotypes: American type and European type. The antigen typing is generally consistent with the respective genotype. The two genotypes have relatively significant differences in antigen types.
The PRRS epidemic caused enormous economic loss to world pork industry. Therefore, prevention of PRRS has received an increasing attention from practitioners worldwide. Vaccine preventions have made certain progress. Inactivated vaccines, which are made during the early development stage, do not involve risks in virus shedding or return of virulence. However, inactivated vaccines cannot provide ideal protection against PRRSV which is primarily eliminated through cell-mediated immunity.
In 1995, Boehringer Ingleheim in the United States provided for the first time an attenuated live vaccine called Resp PRRS/Repro™, which was approved for immunization in 3-18 week old healthy young sows, multiparous sows, suckling piglets, and wean-to-finish pigs for an immunization period of above 4 months. Another attenuated vaccine Prime Pac® PRRS, manufactured by Schering-Plough Animal Health, is used for preventative inoculation in sows or gilts at 3-6 weeks before breeding, and it has shown good preventative effects. However, as PRRSV is prone to mutate, the attenuated vaccines cannot provide desirable preventative clinical effects, which can be the main reason for reoccurrence of PRRS in pig farms.
In 2006, China was hit by an outbreak of highly pathogenic porcine reproductive and respiratory syndrome, and the causative pathogen of this epidemic was a mutated PRRSV strain, which contained a significant extent of mutations as compared to PRRS virus strains found in previous outbreaks. Therefore, there exist needs to develop attenuated live vaccines which specifically target the virus strains causing the epidemic so as to effectively prevent the spread of PRRS.